The present invention relates to optical fibers and more particularly to stress-induced birefringent single mode optical fibers.
A single-mode optical waveguide with a core of circular cross section supports the HE.sub.11 hybrid mode. The polarization of this mode can be resolved into two orthogonal components. When the circularity of the core is perfect, and is maintained along the fiber length, the propagation coefficients of the modes in the two orthogonal directions are exactly the same. The polarization of the mode as it propagates along the fiber should be retained, i.e., at the fiber output the polarization of the mode should be predominantly in one direction if it is so launched at the input. However, it has been observed experimentally that the output polarization of a nominally circular fiber injected by a linearly polarized light is unpredictable due to minor irregularities in the fiber and to changes in environmental conditions. This is because inherent birefringence removes the degeneracy between the two orthogonal polarizations of the fundamental mode, and any strain or imperfections which are either built into the fiber or introduced by bending, twisting or mounting, will lead to power coupling between these two polarizations. Therefore, the state of polarization at the fiber output is arbitrary and can, in fact, vary with time in response to temperature and pressure changes along the fiber length.
This variation with time of the state of polarization becomes a serious problem in connectoring of single mode fibers and polarization sensitive devices, such as optical multiplexers, switches, and fiber acoustic and inertial rotation sensors.
The general approach to maintain linear polarization in a single mode fiber is to increase fiber birefringence to reduce coupling of power between the two polarizations.
In earlier fiber work, breaking the circular symmetry of the core to remove the degeneracy of the two polarizations of the fundamental mode was suggested and observed. Recently, however, it has been reported that only slight improvement in polarization performance is observed using fibers having extremely elliptical cores. It has also been suggested that anisotropic strain is a more dominant factor than noncircular geometry for maintaining linear polarization over long fiber length and that geometrical ellipticities in a dielectric waveguide have only a minor effect on the difference in propagation constant for the two modes of polarization. Therefore, strain birefringence must be introduced to raise the difference in propagation constants.
Birefringent single mode fibers which maintain linear polarization have been made. The birefringence was introduced by breaking the circular symmetry of the strain in the elliptical optical cladding. This anisotropic cladding strain was produced by total or partial flat grinding of the silica on two sides of a single mode preform. During drawing, surface tension causes the fiber to be circular, resulting in an elliptical cladding and an almost circular core. The high silica core remains essentially round as a result of the high softening point. The birefringence depends not only on the cladding ellipticity, but also on the thermal expansion coefficients of the cladding and jacketing glasses.
Most of the anisotropic strain in the cladding makes only a small contribution to the birefringence of the fiber, since most of the light power is confined to the core, or the portion of the cladding immediately adjacent to the core. This is especially true for high NA (numerical aperture) fibers. To further increase birefringence, single mode fibers with two optical claddings have been developed. In the two cladding fibers, the strain is concentrated closer to the core by increasing the boron concentration closer to the core and by decreasing the concentration in the rest of the cladding.
The two cladding fiber was the best fiber for maintaining linear polarization, and was also the most birefringent fiber. When light was injected along either principal axis, the light output from this fiber was better than 99% after 70 meters. No significant depolarization was observed during twisting the fiber several times and bending it to a radius of a few millimeters. The birefringence in this fiber, expressed as a refractive index difference between the two axes was .delta..eta.=4.3.times.10.sup.-5. Fibers with .delta..eta.=1.times.10.sup.-5 will maintain polarization only if handled with extreme care.
The elliptical cladding is extremely useful in locating the principal axes, in addition to causing the birefringence.
It has been observed that a single cladding birefringent single mode fiber maintained polarization when bent, but depolarization was observed when the fiber was twisted. In a two cladding single mode fiber, however, polarization was maintained under bending and twisting conditions.